This invention relates to a screen of a projection television system for projecting an image from projecting tubes thereon, and more particularly to a projection screen of a projection television system in which projecting tubes of red (R), green (G) and blue (B) are aligned in horizontal direction with a convergence adjustment circuit for correcting a chromatic or color deviation generated in a horizontal direction due to the difference in projection angle among the projecting tubes.
In a conventional convergence adjustment circuit for a projection television system, a correction is effected for sub-polarizing yokes for the horizontal direction (H) and the vertical direction (V) by causing a current, corresponding to the chromatic deviation, to flow through the yokes.
Conventionally, an adjuster confirms a distortion pattern of a raster while watching the projection screen, thereby adjusting the current supplied to the sub-polarizing yokes by controlling a volume switch, to correct the chromatic deviation. According to this manual convergence adjustment, there would be a non-uniformity in the adjustment due to the difference in personal adjusting ability and visual ability.
For obviating the non-uniformity, it is desirable to effect an automatic convergence adjustment. As shown in FIG. 7 in a conventional automatic convergence adjustment, a photo receptor 32 with a photo-sensitive elements it moved in a horizontal direction above a surface of a screen 31, so that the convergence adjustment circuit is driven by a signal from the photo-sensitive elements.
In the conventional automatic convergence adjustment, the adjustment must be performed after automatically or manually moving the photo receptor 32 at a preferable position on the screen, and the photo receptor 32 must be moved at the edge of the screen after performing the adjustment. Accordingly, the conventional automatic convergence adjustment suffers problems, e.g. troublesome operations, complicated structures and increased costs.